Count Primes

Count the number of prime numbers less than a non-negative number, n.

思路： 移除每个质数的整数倍。

public class Solution {
    public int countPrimes(int n) {
        if(n <= 0)
            return 0;
        boolean[] isComposite = new boolean[n];
        int cur = 2;
        int count = 0;
        while(cur < n){
            count++;
            moveOutComposite(cur, isComposite);
            cur++;
            while(cur < n && isComposite[cur]){
                cur++;
            }
        }
        return count;
    }
    
    //Move of every current number's composite
    private void moveOutComposite(int cur, boolean[] isComposite){
        for(int i = cur + cur; i < isComposite.length; i += cur){
            isComposite[i] = true;
        }
    }
}